The present invention relates generally to a touch panel apparatus for detecting a touch condition of a finger of an operator on a touch panel and, more particularly, to a real contact type touch panel apparatus in which a finger or a conductor ohmically-contacts a conductive surface of a touch panel.
A touch panel apparatus which utilizes a resistive layer can be generally classified into three groups by a panel structure thereof. The panel structure comprises two resistive layers in conductive relationship with each other, or an array to be partially shaded by a finger. Furthermore, Japanese Patent Nos. 1536723 (William Pepper, Jr., Feb. 22, 1980) and 2603986 (Asano et al. Mar. 11, 1988) disclose touch panel apparatuses having a simple structure of a panel, which basically comprises a single resistive layer.
Japanese Patent Nos. 1536723 and 2603986, disclose touch panel apparatuses directed to both a capacitive coupling touch panel apparatus and a real contact touch apparatus. However, the two patents do not disclose three points which will be described hereinafter. That is, according to the two patents, only the capacitive coupling touch panel apparatus could be easily practiced.
The first point is the problem that noise is generated by a conductive human body. The human body has noise conductivity in a wide range of frequencies. For example, when a human is immediately in front of a CRT display apparatus and a personal computer, he or she receives voltage noise at a much higher level than the signal level generally used in a touch panel. In particular, a 50 to 60 Hz component from a commercial power often exceeding 20 V. The electrostatic capacity coupling type touch panel apparatus has the feature that low frequency noise is automatically eliminated through a coupling capacity of approximately 10 pF. On the other hand, the real contact type touch panel has noise mixed into the original signal, with respect to all frequency components.
The second point is the problem of insulating a finger at a contact location. Particularly, since a power source of a touch panel apparatus is applied from a commercial power of 100 to 200 V, few or large amount of power component (common mode component) is flown into the apparatus through power transfer, electromagnetic coupling, or a power source noise filter (particularly via a capacitor). Therefore, the real contact type touch panel apparatus has a powered part of a circuit on which a finger really contacts. In the electrostatic capacity coupling apparatus, the problem of insulation is not caused, since an insulating layer is disposed on the panel surface.
The third point is the problem that the surface resistive film has no durability. Since the resistive film of the real contact type touch panel apparatus is directly rubbed by a finger, the resistive film must be carefully chosen from durable materials which are selected for the resistance membrane.
Moreover, there is also a problem in the electrostatic capacity coupling type touch panel apparatus, as illustrated in FIG. 5. Referring to FIG. 5, an electrostatic capacity coupling type touch panel apparatus having a surface resistor 54 with uniformity, the surface being coated with an insulation layer, and a low resistance electrode 55 surrounding the surface resistor 54, is illustrated. The four corners A, B, C, and D form a connecting terminal for connecting an electric wire, respectively. The touch panel is touched by a finger 7 of an operator in FIG. 5.
The operator tries to touch and indicate a point P1 on the touch panel with his finger 7. However, coupling capacity also will be generated between other fingers besides the finger 7, and points P2 and P3 on the touch panel, respectively. In addition, coupling capacity will be generated between his palm and a point P4. Accurately, these additional coupling capacities distribute about the points P2, P3, and P4 on the touch panel. The touch panel apparatus will identify a point Pm determined by calculating a mean point between P1, P2, P3, and P4 on the touch panel. Accordingly, a final input point for the touch panel apparatus will be at a location which is shifted in the direction of the user""s palm. The shift phenomenon of the point Pi on the touch panel will be hereinafter called xe2x80x9ca palm effectxe2x80x9d. The palm effect can influence all types of electrostatic capacity coupling type touch panels with use of a uniform resistive film, including the touch panel apparatus with the construction illustrated in FIG. 5.
The subject of the present invention is directed to categories of touch panel apparatuses which utilize resistive film comprising a single layer. In addition, the subject of the present invention includes uncovering uncertain reasons of the grounding effect of human body.
In order to avoid the palm effect, the present invention provides a touch panel apparatus having a resistive film on a touch panel to be directly contacted by a finger.
Secondly, in order to avoid the harmful influence of noise, the present invention provides a touch panel apparatus comprising a signal processing unit having an input circuit provided with a current/voltage conversion circuit. The current/voltage conversion circuit includes a transistor having a grounded base or gate, an emitter into which an AC signal is input, and a collector or drain electrically connected to a resonance circuit comprising a coil and a capacitor. The touch panel apparatus further comprises signal level measuring means for improving a S/N ratio of a detection signal and effecting a high resolution A/D conversion. The signal level measuring means comprises A/D converting means, the A/D converting means including means for superimposing a step-shaped triangular wave having an amplitude of 2 or more LSB of an analog/digital converter, on an output voltage level of AC/DC (AM detection), and means for adding the output at each step of the step-shaped triangular wave from the analog/digital converter.
Thirdly, in order to assure an insulation of a panel in the touch panel apparatus, a voltage-floating system (a circuit block) is provided in the touch panel apparatus. Particularly, a panel unit and the signal processing unit in the touch panel apparatus are also arranged in the voltage-floating system. The voltage-floating system enables reception and transmission of analog or digital electric information from and to a non-floating system through an isolator. The voltage-floating system has a reference potential circuit connected to a ground circuit of the non-floating system through a series circuit consisting of an AC signal generator and an AC signal coupling capacitor of 5000 pF or less.
Fourthly, in consideration of the durability of a resistive film on a panel, the resistive film comprises tin oxide.
Fifthly, in order to obtain the grounding effect of human body unconditionally, an AC signal current flown between a finger and a panel is set at frequency of 200 kHz or more.
In accordance with a first aspect of the present invention, contact resistance between a portion of contact of a finger which touches a panel and a location of a panel touched by the finger is utilized. The contact resistance is set to a much smaller value than the magnitude of capacitive coupling impedances of a palm and the like so that approximately 100% of the AC current passes through the location on the panel touched by the finger. Accordingly, when the touch panel apparatus senses the location on the panel touched by the finger, the input location is not shifted by influence of the palm effect.
In a second aspect of the invention, some key points are exemplified. A first point is that since a noise source generally has a very high impedance, even though the voltage is high, a large amount of current is not drawn. This is one of the reasons that signals from a panel are detected by means of current detection in the apparatus. A second point is that even though either a bipolar transistor or a field effect transistor is used, a sufficient linearity to be required is produced. Because 99.5% (the bipolar transistor) or 99.95% (field effect transistor) of an emitter or source input current is sent to a collector or drain, the linearity of a circuit can be maintained until instantaneous current at a collector or drain has reached approximately zero. Since the collector or drain undergoes a load applied from a resonance circuit which comprises a coil and a capacitor, even though very large efficiency of current/voltage conversion at a signal frequency is obtained, a voltage on the collector or drain does not undergo a voltage change and is not saturated. Accordingly, the touch panel apparatus has a considerably large noise-margin. In addition, samples are picked up n times, n being an integer, and the picked-up samples are averaged. Consequently, a S/N ratio is improved by {square root over (n)} times. In the case of a panel using a resistive surface, since a location touched by a finger is calculated in accordance with a ratio of signal levels detected from some points on the panel, it is not required to average the detected signals. According to the present invention, each of the outputs of an analog/digital converter are only addedn times so that the S/N ratio is automatically improved. Moreover, at this time, if a step-shaped triangular wave is superimposed on the measured analog voltage, although A/D linearity is not increased, equivalently improved resolution of A/D conversion is also provided through the superimposed wave.
A third aspect of the invention is based on a capacitor having an impedance depending on frequency. The capacitor has sufficiently low impedance at AC signal frequencies for providing an operation of a coupling capacitor, while it has a high impedance at a power source frequency range of 50 to 60 Hz, which is a value regarded as an insulating material.
In a fourth aspect, a tin oxide-based ceramic film provides mechanical or chemical features of excellent durability similarly to glass, and more effective lubricity than glass.
In a fifth aspect of the invention, a human body having a resistance of a few to approximately 10 kxcexa9 as a conductor provides an antenna to cause electromagnetic wave to be radiated, when the human body (conductor) is excited by AC voltage. In general, the radiant impedance as the antenna is reduced, if higher exciting-frequency is supplied from the touch panel apparatus. The voltage oscillation of the human body (conductor) is loaded with the radiant impedance. Therefore, even though the human body is sitting on an insulated chair or standing on an insulated stand, the voltage-restraining effect (the ground effect) on the human body provides preferable performance for oscillation frequency (the AC signal frequency) of 200 or more kHz.
The present invention resides in a real (or direct) contact type touch panel system for inputting a location (coordinates) on a panel that a point on the panel is indicated with a finger of an operator or a conductor. The present invention also resides in a real contact type touch panel apparatus for inputting a position indicating a section on a panel with a finger of an operator or a conductor as used in a touch keyboard, and a real contact type touch panel apparatus for detecting only a contact of a finger or a conductor, or a touch panel apparatus for providing all of the functions described above.